The use of solder mask material in the formation of printed circuit boards (hereinafter also referred to as PCB's), chip carriers and other circuitized substrate products is well known. Solder mask is typically applied over the substrate's exposed surface circuitry prior to the components being soldered to the substrate, and may be applied to one or both exposed surfaces (the latter if opposite sides are being populated with components). Solder mask materials include the photoimagable type, sometimes in liquid form, with various examples including the Valu-SMT® series of materials sold by E.I. duPont de Nemours and Company and the Probimer.® solder mask series of materials sold by Huntsman Advanced Materials Corporation. As is known, these materials provide dielectric and mechanical protection of the circuitry during and after soldering operations. Typically, the solder mask may be applied uniformly over the circuitry (e.g., copper “traces”) in single layer form, using curtain coating, screen coating, spray coating and roller coating procedures, all of which are known in the art as well as the various apparatus for performing same. A mesh screen may be utilized for screen coating, for example. A substrate which has been coated with the single layer of liquid photoimagable solder mask is then typically set into a chamber, where it is exposed to ultraviolet (UV) light which cures the liquid material. Preferably, the cured solder mask exhibits no visible cracks, scratches, or other surface defects even after the soldering processes which attach either the surface mount technology (SMT) components or the plated through hole (PTH) components to the substrate's upper surface.
It is necessary that the solder mask material be formulated such that it possesses rheological properties for effective coating. Further, the solder mask must permit efficient transmission of the light or other (e.g., UV) exposing radiation so as to photolyze the photoinitiator through whatever thickness of material is applied. Also, of course, the solder mask must possess appropriate physical and chemical properties to withstand the application of the solder without significant deterioration or degradation and maintain its coverage over the portions of the board wherein solder is to be masked.
Conventional photoimageable solder mask materials, including those of the aforementioned liquid type, typically contain about 30-80% solids in solvents, such as naphtha, propylene glycol monomethyl ether, diglyme, and/or propylene glycol monomethyl ether acetate. These solder mask materials also require organic solvents to apply the mask to a substrate and to develop the deposited material. Some of these organic solvents tend to evaporate and are lost to the environment. Of the total solvent lost to the environment, approximately 85-95% is typically lost during the application process and another approximately 5-15% is lost during the developing steps. Of more recent vintage, solder mask materials have been devised that are designated “aqueous” solder mask materials. While these are developed using aqueous solvents, these are nevertheless applied using organic solvents. The use of such aqueous solder masks reduces the solvent loss associated with the non-aqueous types by only about 5-15%. In contrast, a solder mask that is applied using an aqueous solvent, even though developed using an organic solvent, would reduce the overall organic solvent emissions by about 85-95%.
Examples of various photoimageable solder mask materials and various processes involving the use of solder masks are described in the following U.S. Letters Patents.
In U.S. Pat. No. 6,210,862, there is described a photoimagable cationically polymerizable epoxy based solder mask that contains a non-brominated epoxy resin system and from about 0.1 to about 15 parts, by weight per 100 parts of resin system, of a cationic photoinitiator. The non-brominated epoxy-resin system has solids that are comprised of from about 10% to about 80% by weight, of a polyol resin having epoxy functionality; from about 0% to about 90% by weight of a polyepoxy resin; and from about 25% to about 85% by weight of an difunctional epoxy resin. Since the photosensitive cationically polymerizable epoxy based solder mask does not contain bromine, it is particularly advantageous in waste processing in incineration of waste circuit boards. The photosensitive cationically polymerizable non-brominated epoxy based solder mask has a glass transition temperature greater than about 100.degrees C., and preferably greater than about 110.degrees C. The solder mask dries to a tack-free film; thus, artwork used in the photoimaging process will not stick to the dried soldermask film. The polyol resin which is a condensation product of epichlorohydrin and bisphenol A, has a weight average molecular weight of between about 40,000 and 130,000. The polyepoxy resin is an epoxidized multi-functional bisphenol A formaldehyde novolak resin having a weight average molecular weight of 4,000 to 10,000. The epoxidized diglycidyl ether of bisphenol A has two epoxide groups per molecule, a melting point of between about 80.degrees C. and about 110.degrees C. and a weight average molecular weight of between about 600 and 2,500.
In U.S. Pat. No. 5,863,597, there is described a cost-effective process for preventing mealing of a polyurethane conformal coating applied to a printed wiring board is described. The process includes utilizing a liquid photo-imagable solder mask for protecting the circuitry within the board prior to applying the polyurethane conformal coating. The process further includes applying the polyurethane coating within a controlled maximum thickness of 0.0015 inches for preventing vesication of the conformal coating.
In U.S. Pat. No. 5,798,285, there is described the application of a first plating resist for forming circuit lines on a carrier substrate. While the plating resist is still in place, a metal, such as nickel, is deposited on top of the circuit lines. A second plating resist is employed for plating solder on the circuit lines at solder sites. At this stage, additional solder can be deposited at each solder site to provide or supplement the necessary low melt solder required for forming a solder joint. The first and second resists along with solder thereon are then stripped and copper foil on the carrier substrate is etched away around the circuit lines. A solder mask is then formed on the carrier substrate over the circuit lines except for circuit lines in the chip sites. The solder mask has a single large opening at each chip site which has lateral dimensions which are slightly larger than the lateral dimensions of the chip to be connected at the chip site. During curing of the solder mask, which involves heat, the nickel layer on top of the circuit lines within the chip site opening quickly oxidizes to provide solder dams which extend along the lengths of the lines within the chip site openings immediately adjacent the solder sites. The chips are then placed within the solder mask windows and electrically connected by solder joints to the solder sites of the circuit lines by a flip chip attach method. The chip sites are then encapsulated with an under-fill encapsulant to protect the solder joints.
In U.S. Pat. No. 5,439,779, there is described a photoimageable solder mask of the aforementioned aqueous type; that is, one that may be applied using aqueous solvents, thereby reducing the emission of organic solvents. The solder mask contains an epoxy based resin system comprised of at least one epoxy resin, a coating agent, and preferably a cationic photoinitiator and a dye.
In U.S. Pat. No. 5,300,402, there is described a photoimagable cationically polymerizable epoxy based coating material which includes an epoxy resin system consisting essentially of between about 10% and about 80% by weight of a polyol resin which is a condensation product of epichlorohydrin and bisphenol A having a molecular weight of between about 40,000 and 130,000; between about 20% and about 90% by weight of an epoxidized octafunctional bisphenol A formaldehyde novolak resin having a molecular weight of 4,000 to 10,000; and if flame retardancy is required, between about 35% and 50% by weight of an epoxidized glycidyl ether of tetrabromo bisphenol A having a softening point of between about 60 C. and about 110 C. and a molecular weight of between about 600 and 2,500. To this resin system is added about 0.1 to about 15 parts by weight per 100 parts of resin of a cationic photoinitiator capable of initiating polymerization of said epoxidized resin system upon exposure to actinic radiation; the system being further characterized by having an absorbance of light in the 330 to 700 nm region of less than 0.1 for a 2.0 mil thick film, and an effective amount of thixatropic agent. Optionally a photosensitizer such as perylene and its derivatives or anthracene and its derivatives may be added.
In U.S. Pat. No. 4,668,603, a solder mask is first applied to a printed circuit board which contains a raised relief due to the conductive areas and nonconductive areas, the solder mask being a negative acting solder mask layer and applied using vacuum lamination. Thereafter, the photosensitive solder mask film is image-wise exposed to actinic radiation with unexposed areas of the film washed away to expose portions of the substrate which are conductive. Thereafter a substrate surface typically is exposed to a molten wave of solder with solidification onto exposed conductive portions of the substrate without obtaining any adhering coating on the solder mask.
In U.S. Pat. No. 4,624,912, there is described a process for the production of a protective layer or a relief image on a substrate, wherein a radiation-sensitive layer, consisting of a solid film-forming epoxy resin containing a photoinitiator, which can be activated by radiation, for the polyaddition reaction, is transferred from a support to a substrate, then exposed directly or under a photomask and hardened by the action of heat, after which, if appropriate, the unexposed parts are developed with a solvent. The process is suitable, for example, for the production of printed circuits, solder resist masks and offset printing plates.
In U.S. Pat. No. 4,506,004, there is described the application of a two layer coating of solder mask. One inner adhesive photopolymer layer is applied to a PCB in the liquid state, displacing air from the PCB's surface. The outer layer of the composite is dry and is carried on a thin plastic sheet and over-laminated onto the liquid inner layer, without the need for a vacuum laminator. The dry film solder mask so laminated is then exposed through a photo-transparency to harden the light struck dry film solder mask and light struck inner layer photopolymer, thereby co-joining the dry film solder mask, inner layer and PCB surface. A solvent washout step removes unexposed dry film solder mask and unexposed inner layer photopolymer. The composite coating can be a combination of known solder mask materials, dry film, UV-curable and thermal-curing epoxy. U.S. Pat. No. 4,966,827 is similar in such teachings.
In U.S. Pat. No. 4,487,828, there is described a method for manufacturing a printed wiring board having a solder layer existing on circuit paths located on surfaces of the board and a solder layer on the walls of thru holes located in the board. The method involves coating the surface of the board with a layer of photopolymer resist. A shield is placed over selected areas of the solder layer on the surface and the shielded surface is exposed to ultra violet light until the unshielded resist hardens. The shielded resist which has remained soft is removed and the board is washed with solder resist to remove the solder layer not covered by a coating of hardened resist.
Finally, in U.S. Pat. No. 3,628,999, there is described a method for producing printed circuit boards which includes the application of a temporary, strippable solder mask together with a permanent solder mask.
Understandably, there are several other photoimageable solder mask materials and processes utilizing same known in the art than those described above. The above listing is, therefore, not meant to be exhaustive.
It has been determined that the single layer application of liquid photoimageable solder mask material to circuit patterns of high density may result in a failure of the material on some occasions to adequately contact some parts of the circuit (especially those of “taller” configurations). This is believed the result of various factors, including the aforementioned solids in the solvent, dyes and pigments (for imparting desired color), the mesh of the screen utilized, various flow modifiers, etc. For circuit patterns of lesser density, this has not often presented a serious problem. However, for high-density circuit patterns required in many of today's products, a more satisfactory procedure is deemed necessary.
It is believed, therefore, that a process for applying photoimageable solder mask material in a manner that will assure effective coverage of circuit elements in a high-density pattern of such elements of different heights (thicknesses) would represent a significant advancement in the art.